Network Working Group B. Rajagopalan
Request for Comments: 3717 Consultant
Category: Informational J. Luciani
Marconi Communications
D. Awduche
MCI
March 2004
IP over Optical Networks: A Framework
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
The Internet transport infrastructure is moving towards a model of
high-speed routers interconnected by optical core networks. The
architectural choices for the interaction between IP and optical
network layers, specifically, the routing and signaling aspects, are
maturing. At the same time, a consensus has emerged in the industry
on utilizing IP-based protocols for the optical control plane. This
document defines a framework for IP over Optical networks,
considering both the IP-based control plane for optical networks as
well as IP-optical network interactions (together referred to as "IP
over optical networks").
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RFC 3717 IP over Optical Networks: A Framework March 20041. Introduction
Optical network technologies are evolving rapidly in terms of
functions and capabilities. The increasing importance of optical
networks is evidenced by the copious amount of attention focused on
IP over optical networks and related photonic and electronic
interworking issues by all major network service providers,
telecommunications equipment vendors, and standards organizations. In
this regard, the term "optical network" is used generically in
practice to refer to both SONET/SDH-based transport networks, as well
as switched optical networks (including all-optical networks).
It has been realized that optical networks must be survivable,
flexible, and controllable. There is, therefore, an ongoing trend to
introduce intelligence in the control plane of optical networks to
make them more versatile [1]. An essential attribute of intelligent
optical networks is the capability to instantiate and route optical
layer connections in real-time or near real-time, and to provide
capabilities that enhance network survivability. Furthermore, there
is a need for multi-vendor optical network interoperability, when an
optical network may consist of interconnected vendor-specific optical
sub-networks.
The optical network must also be versatile because some service
providers may offer generic optical layer services that may not be
client-specific. It would therefore be necessary to have an optical
network control plane that can handle such generic optical services.
There is general consensus in the industry that the optical network
control plane should utilize IP-based protocols for dynamic
provisioning and restoration of optical channels within and across
optical sub-networks. This is based on the practical view that
signaling and routing mechanisms developed for IP traffic engineering
applications could be re-used in optical networks. Nevertheless, the
issues and requirements that are specific to optical networking must
be understood to suitably adopt and adapt the IP-based protocols.
This is especially the case for restoration, and for routing and
signaling in all-optical networks. Also, there are different views
on the model for interaction between the optical network and client
networks, such as IP networks. Reasonable architectural alternatives
in this regard must be supported, with an understanding of their
relative merits.
Thus, there are two fundamental issues related to IP over optical
networks. The first is the adaptation and reuse of IP control plane
protocols within the optical network control plane, irrespective of
the types of digital clients that utilize the optical network. The
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RFC 3717 IP over Optical Networks: A Framework March 2004
second is the transport of IP traffic through an optical network
together with the control and coordination issues that arise
therefrom.
This document defines a framework for IP over optical networks
covering the requirements and mechanisms for establishing an IP-
centric optical control plane, and the architectural aspects of IP
transport over optical networks. In this regard, it is recognized
that the specific capabilities required for IP over optical networks
would depend on the services expected at the IP-optical interface as
well as the optical sub-network interfaces. Depending on the
specific operational requirements, a progression of capabilities is
possible, reflecting increasingly sophisticated interactions at these
interfaces. This document therefore advocates the definition of
"capability sets" that define the evolution of functionality at the
interfaces as more sophisticated operational requirements arise.
This document is organized as follows. In the next section,
terminology covering some basic concepts related to this framework
are described. The definitions are specific to this framework and
may have other connotations elsewhere. In Section 3, the network
model pertinent to this framework is described. The service model
and requirements for IP-optical, and multi-vendor optical
internetworking are described in Section 4. This section also
considers some general requirements. Section 5 considers the
architectural models for IP-optical interworking, describing the
relative merits of each model. It should be noted that it is not the
intent of this document to promote any particular model over the
others. However, particular aspects of the models that may make one
approach more appropriate than another in certain circumstances are
described. Section 6 describes IP-centric control plane mechanisms
for optical networks, covering signaling and routing issues in
support of provisioning and restoration. The approaches described in
Section 5 and 6 range from the relatively simple to the
sophisticated. Section 7 describes a number of specialized issues in
relation to IP over optical networks. Section 8 describes a possible
evolution path for IP over optical networking capabilities in terms
of increasingly sophisticated functionality that may be supported as
the need arises. Section 9 considers security issues pertinent to
this framework. Finally, the summary and conclusion are presented in
Section 10.
2. Terminology and Concepts
This section introduces terminology pertinent to this framework and
some related concepts. The definitions are specific to this
framework and may have other interpretations elsewhere.
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RFC 3717 IP over Optical Networks: A Framework March 2004
WDM
Wavelength Division Multiplexing (WDM) is a technology that allows
multiple optical signals operating at different wavelengths to be
multiplexed onto a single optical fiber and transported in parallel
through the fiber. In general, each optical wavelength may carry
digital client payloads at a different data rate (e.g., OC-3c, OC-
12c, OC- 48c, OC-192c, etc.) and in a different format (SONET,
Ethernet, ATM, etc.). For example, there are many commercial WDM
networks in existence today that support a mix of SONET signals
operating at OC-48c (approximately 2.5 Gbps) and OC-192
(approximately 10 Gbps) over a single optical fiber. An optical
system with WDM capability can achieve parallel transmission of
multiple wavelengths gracefully while maintaining high system
performance and reliability. In the near future, commercial dense
WDM systems are expected to concurrently carry more than 160
wavelengths at data rates of OC-192c and above, for a total of 1.6
Tbps or more. The term WDM will be used in this document to refer to
both WDM and DWDM (Dense WDM).
In general, it is worth noting that WDM links are affected by the
following factors, which may introduce impairments into the optical
signal path:
1. The number of wavelengths on a single fiber.
2. The serial bit rate per wavelength.
3. The type of fiber.
4. The amplification mechanism.
5. The number and type of nodes through which the signals pass before
reaching the egress node or before regeneration.
All these factors (and others not mentioned here) constitute domain
specific features of optical transport networks. As noted in [1],
these features should be taken into account in developing standards
based solutions for IP over optical networks.
Optical cross-connect (OXC)
An OXC is a space-division switch that can switch an optical data
stream from an input port to a output port. Such a switch may
utilize optical-electrical conversion at the input port and
electrical-optical conversion at the output port, or it may be all-
optical. An OXC is assumed to have a control-plane processor that
implements the signaling and routing protocols necessary for
computing and instantiating optical channel connectivity in the
optical domain.
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RFC 3717 IP over Optical Networks: A Framework March 2004
Optical channel trail or Lightpath
An optical channel trail is a point-to-point optical layer connection
between two access points in an optical network. In this document,
the term "lightpath" is used interchangeably with optical channel
trail.
Optical mesh sub-network
An optical sub-network, as used in this framework, is a network of
OXCs that supports end-to-end networking of optical channel trails
providing functionality like routing, monitoring, grooming, and
protection and restoration of optical channels. The interconnection
of OXCs in this network can be based on a general mesh topology. The
following sub-layers may be associated with this network:
(a) An optical multiplex section (OMS) layer network: The optical
multiplex section layer provides transport for the optical
channels. The information contained in this layer is a data
stream comprising a set of optical channels, which may have a
defined aggregate bandwidth.
(b) An optical transmission section (OTS) layer network: This layer
provides functionality for transmission of optical signals
through different types of optical media.
This framework does not address the interaction between the optical
sub-network and the OMS, or between the OMS and OTS layer networks.
Mesh optical network (or simply, "optical network")
A mesh optical network, as used in document, is a topologically
connected collection of optical sub-networks whose node degree may
exceed 2. Such an optical network is assumed to be under the purview
of a single administrative entity. It is also possible to conceive
of a large scale global mesh optical network consisting of the
voluntary interconnection of autonomous optical networks, each of
which is owned and administered by an independent entity. In such an
environment, abstraction can be used to hide the internal details of
each autonomous optical cloud from external clouds.
Optical internetwork
An optical internetwork is a mesh-connected collection of optical
networks. Each of these networks may be under a different
administration.
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RFC 3717 IP over Optical Networks: A Framework March 2004
Wavelength continuity property
A lightpath is said to satisfy the wavelength continuity property if
it is transported over the same wavelength end-to-end. Wavelength
continuity is required in optical networks with no wavelength
conversion feature.
Wavelength path
A lightpath that satisfies the wavelength continuity property is
called a wavelength path.
Opaque vs. transparent optical networks
A transparent optical network is an optical network in which optical
signals are transported from transmitter to receiver entirely in the
optical domain without OEO conversion. Generally, intermediate
switching nodes in a transparent optical network do not have access
to the payload carried by the optical signals.
Note that amplification of signals at transit nodes is permitted in
transparent optical networks (e.g., using Erbium Doped Fiber
Amplifiers << EDFAs).
On the other hand, in opaque optical networks, transit nodes may
manipulate optical signals traversing through them. An example of
such manipulation would be OEO conversion which may involve 3R
operations (reshaping, retiming, regeneration, and perhaps
amplification).
Trust domain
A trust domain is a network under a single technical administration
in which adequate security measures are established to prevent
unauthorized intrusion from outside the domain. Hence, it may be
assumed that most nodes in the domain are deemed to be secure or
trusted in some fashion. Generally, the rule for "single"
administrative control over a trust domain may be relaxed in practice
if a set of administrative entities agree to trust one another to
form an enlarged heterogeneous trust domain. However, to simplify
the discussions in this document, it will be assumed, without loss of
generality, that the term trust domain applies to a single
administrative entity with appropriate security policies. It should
be noted that within a trust domain, any subverted node can send
control messages which can compromise the entire network.
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RFC 3717 IP over Optical Networks: A Framework March 2004
Flow
In this document, the term flow will be used to signify the smallest
non-separable stream of data, from the point of view of an endpoint
or termination point (source or destination node). The reader should
note that the term flow is heavily overloaded in contemporary
networking literature. In this document, we will consider a
wavelength to be a flow, under certain circumstances. However, if
there is a method to partition the bandwidth of the wavelength, then
each partition may be considered a flow, for example using time
division multiplexing (TDM), it may be feasible to consider each
quanta of time within a given wavelength as a flow.
Traffic Trunk
A traffic trunk is an abstraction of traffic flow traversing the same
path between two access points which allows some characteristics and
attributes of the traffic to be parameterized.
3. The Network Model3.1. Network Interconnection
The network model considered in this memo consists of IP routers
attached to an optical core internetwork, and connected to their
peers over dynamically established switched optical channels. The
optical core itself is assumed to be incapable of processing
individual IP packets in the data plane.
The optical internetwork is assumed to consist of multiple optical
networks, each of which may be administered by a different entity.
Each optical network consists of sub-networks interconnected by
optical fiber links in a general topology (referred to as an optical
mesh network). This network may contain re-configurable optical
equipment from a single vendor or from multiple vendors. In the near
term, it may be expected that each sub-network will consist of
switches from a single vendor. In the future, as standardization
efforts mature, each optical sub-network may in fact contain optical
switches from different vendors. In any case, each sub-network
itself is assumed to be mesh-connected internally. In general, it
can be expected that topologically adjacent OXCs in an optical mesh
network will be connected via multiple, parallel (bi-directional)
optical links. This network model is shown in Figure 1.
In this environment, an optical sub-network may consist entirely of
all-optical OXCs or OXCs with optical-electrical-optical (OEO)
conversion. Interconnection between sub-networks is assumed to be
implemented through compatible physical interfaces, with suitable
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RFC 3717 IP over Optical Networks: A Framework March 2004
optical-electrical conversions where necessary. The routers that
have direct physical connectivity with the optical network are
referred to as "edge routers" with respect to the optical network. As
shown in Figure 1, other client networks (e.g., ATM) may also connect
to the optical network.
The switching function in an OXC is controlled by appropriately
configuring the cross-connect fabric. Conceptually, this may be
viewed as setting up a cross-connect table whose entries are of the
form <input port i, output port j>, indicating that the data stream
entering input port i will be switched to output port j. In the
context of a wavelength selective cross-connect (generally referred
to as a WXC), the cross-connect tables may also indicate the input
and output wavelengths along with the input and output ports. A
lightpath from an ingress port in an OXC to an egress port in a
remote OXC is established by setting up suitable cross-connects in
the ingress, the egress and a set of intermediate OXCs such that a
continuous physical path exists from the ingress to the egress port.
Optical paths tend to be bi-directional, i.e., the return path from
the egress port to the ingress port is typically routed along the
same set of intermediate interface cards as the forward path, but
this may not be the case under all circumstances.
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RFC 3717 IP over Optical Networks: A Framework March 2004
the data stream received on wavelength Lambda(j) over input port i is
switched to output port k on Lambda(l). Automated establishment of
lightpaths involves setting up the cross-connect table entries in the
appropriate OXCs in a coordinated manner such that the desired
physical path is realized.
Under this network model, a switched lightpath must be established
between a pair of IP routers before the routers can transfer user
traffic among themselves. A lightpath between IP routers may
traverse multiple optical networks and be subject to different
provisioning and restoration procedures in each network.
The IP-based control plane issue for optical networks pertains to the
design of standard signaling and routing protocols for provisioning
and restoration of lightpaths across multiple optical networks.
Similarly, IP transport over optical networks involves establishing
IP reachability and seamlessly constructing forwarding paths from one
IP endpoint to another over an optical network.
3.2. Control Structure
There are three logical control interfaces identified in Figure 1.
These are the client-optical internetwork interface, the internal
node-to-node interface within an optical network (between OXCs in
different sub-networks), and the external node-to-node interface
between nodes in different optical networks. These interfaces are
also referred to as the User-Network Interface (UNI), the internal
NNI (INNI), and the external NNI (ENNI), respectively.
The distinction between these interfaces arises out of the type and
amount of control information flow across them. The client-optical
internetwork interface (UNI) represents a service boundary between
the client (e.g., IP router) and the optical network. The client and
server (optical network) are essentially two different roles: the
client role requests a service connection from a server; the server
role establishes the connection to fulfill the service request --
provided all relevant admission control conditions are satisfied.
Thus, the control flow across the client-optical internetwork
interface is dependent on the set of services defined across it and
the manner in which the services may be accessed. The service models
are described in Section 4. The NNIs represent vendor-independent
standardized interfaces for control flow between nodes. The
distinction between the INNI and the ENNI is that the former is an
interface within a given network under a single technical
administration, while the later indicates an interface at the
administrative boundary between networks. The INNI and ENNI may thus
differ in the policies that restrict control flow between nodes.
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RFC 3717 IP over Optical Networks: A Framework March 2004
Security, scalability, stability, and information hiding are
important considerations in the specification of the ENNI. It is
possible in principle to harmonize the control flow across the UNI
and the NNI and eliminate the distinction between them. On the other
hand, it may be required to minimize flow of control information,
especially routing-related information, over the UNI; and even over
the ENNI. In this case, UNI and NNIs may look different in some
respects. In this document, these interfaces are treated as
distinct.
The client-optical internetwork interface can be categorized as
public or private depending upon context and service models. Routing
information (i.e., topology state information) can be exchanged
across a private client-optical internetwork interface. On the other
hand, such information is not exchanged across a public client-
optical internetwork interface, or such information may be exchanged
with very explicit restrictions (including, for example abstraction,
filtration, etc). Thus, different relationships (e.g., peer or
over-lay, Section 5) may occur across private and public logical
interfaces.
The physical control structure used to realize these logical
interfaces may vary. For instance, for the client-optical
internetwork interface, some of the possibilities are:
1. Direct interface: An in-band or out-of-band IP control channel
(IPCC) may be implemented between an edge router and each OXC to
which it is connected. This control channel is used for
exchanging signaling and routing messages between the router and
the OXC. With a direct interface, the edge router and the OXC it
connects to are peers with respect to the control plane. This
situation is shown in Figure 2. The type of routing and signaling
information exchanged across the direct interface may vary
depending on the service definition. This issue is addressed in
the next section. Some choices for the routing protocol are OSPF
or ISIS (with traffic engineering extensions and additional
enhancements to deal with the peculiar characteristics of optical
networks) or BGP, or some other protocol. Other directory-based
routing information exchanges are also possible. Some of the
signaling protocol choices are adaptations of RSVP-TE or CR-LDP.
The details of how the IP control channel is realized is outside
the scope of this document.
2. Indirect interface: An out-of-band IP control channel may be
implemented between the client and a device in the optical network
to signal service requests and responses. For instance, a
management system or a server in the optical network may receive
service requests from clients. Similarly, out-of-band signaling
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RFC 3717 IP over Optical Networks: A Framework March 2004
1. Lightpath creation: This service allows a lightpath with the
specified attributes to be created between a pair of termination
points in the optical network. Lightpath creation may be subject
to network-defined policies (e.g., connectivity restrictions) and
security procedures.
2. Lightpath deletion: This service allows an existing lightpath to
be deleted.
3. Lightpath modification: This service allows certain parameters of
the lightpath to be modified.
4. Lightpath status enquiry: This service allows the status of
certain parameters of the lightpath (referenced by its ID) to be
queried by the router that created the lightpath.
An end-system discovery procedure may be used over the UNI to verify
local port connectivity between the optical and client devices, and
allows each device to bootstrap the UNI control channel. Finally, a
"service discovery" procedure may be employed as a precursor to
obtaining UNI services. Service discovery allows a client to
determine the static parameters of the interconnection with the
optical network, including the UNI signaling protocols supported.
The protocols for neighbor and service discovery are different from
the UNI signaling protocol itself (for example, see LMP [2]).
Because a small set of well-defined services is offered across the
UNI, the signaling protocol requirements are minimal. Specifically,
the signaling protocol is required to convey a few messages with
certain attributes in a point-to-point manner between the router and
the optical network. Such a protocol may be based on RSVP-TE or LDP,
for example.
The optical domain services model does not deal with the type and
nature of routing protocols within and across optical networks.
The optical domain services model would result in the establishment
of a lightpath topology between routers at the edge of the optical
network. The resulting overlay model for IP over optical networks is
discussed in Section 5.
4.2. Unified Service Model
Under this model, the IP and optical networks are treated together as
a single integrated network from a control plane point of view. In
this regard, the OXCs are treated just like any other router as far
as the control plane is considered. Thus, in principle, there is no
distinction between the UNI, NNIs and any other router-to-router
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RFC 3717 IP over Optical Networks: A Framework March 2004
interface from a routing and signaling point of view. It is assumed
that this control plane is IP-based, for example leveraging the
traffic engineering extensions for MPLS or GMPLS, as described in
[1]. The unified service model has so far been discussed only in the
context of a single administrative domain. A unified control plane
is possible even when there are administrative boundaries within an
optical internetwork, but some of the integrated routing capabilities
may not be practically attractive or even feasible in this case (see
Section 5).
Under the unified service model and within the context of a GMPLS
network, optical network services are obtained implicitly during
end-to-end GMPLS signaling. Specifically, an edge router can create
a lightpath with specified attributes, or delete and modify
lightpaths as it creates GMPLS label-switched paths (LSPs). In this
regard, the services obtained from the optical network are similar to
the domain services model. These services, however, may be invoked
in a more seamless manner as compared to the domain services model.
For instance, when routers are attached to a single optical network
(i.e., there are no ENNIs), a remote router could compute an end-to-
end path across the optical internetwork. It can then establish an
LSP across the optical internetwork. But the edge routers must still
recognize that an LSP across the optical internetwork is a
lightpath, or a conduit for multiple packet-based LSPs.
The concept of "forwarding adjacency" can be used to specify virtual
links across optical internetworks in routing protocols such as OSPF
[3]. In essence, once a lightpath is established across an optical
internetwork between two edge routers, the lightpath can be
advertised as a forwarding adjacency (a virtual link) between these
routers. Thus, from a data plane point of view, the lightpaths
result in a virtual overlay between edge routers. The decisions as
to when to create such lightpaths, and the bandwidth management for
these lightpaths is identical in both the domain services model and
the unified service model. The routing and signaling models for
unified services is described in Sections 5 and 6.
4.3. Which Service Model?
The relative merits of the above service models can be debated at
length, but the approach recommended in this framework is to define
routing and signaling mechanisms in support of both models. As noted
above, signaling for service requests can be unified to cover both
models. The developments in GMPLS signaling [4] for the unified
service model and its adoption for UNI signaling [5, 6] under the
domain services model essentially supports this view. The
significant difference between the service models, however, is in
routing protocols, as described in Sections 5 and 6.
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RFC 3717 IP over Optical Networks: A Framework March 20044.4. What are the Possible Services?
Specialized services may be built atop the point-to-point
connectivity service offered by the optical network. For example,
optical virtual private networks and bandwidth on demand are some of
the services that can be envisioned.
4.4.1. Optical Virtual Private Networks (OVPNs)
Given that the data plane links between IP routers over an optical
network amounts to a virtual topology which is an overlay over the
fiber optic network, it is easy to envision a virtual private network
of lightpaths that interconnect routers (or any other set of clients)
belonging to a single entity or a group of related entities across a
public optical network. Indeed, in the case where the optical
network provides connectivity for multiple sets of external client
networks, there has to be a way to enforce routing policies that
ensure routing separation between different sets of client networks
(i.e., VPN service).
5. IP transport over Optical Networks
To examine the architectural alternatives for IP over optical
networks, it is important to distinguish between the data and control
planes. The optical network provides a service to external entities
in the form of fixed bandwidth transport pipes (optical paths). IP
routers at the edge of the optical networks must necessarily have
such paths established between them before communication at the IP
layer can commence. Thus, the IP data plane over optical networks is
realized over a virtual topology of optical paths. On the other
hand, IP routers and OXCs can have a peer relation with respect to
the control plane, especially for routing protocols that permit the
dynamic discovery of IP endpoints attached to the optical network.
The IP over optical network architecture is defined essentially by
the organization of the control plane. The assumption in this
framework is that an IP-based control plane [1] is used, such as
GMPLS. Depending on the service model(Section 4), however, the
control planes in the IP and optical networks can be loosely or
tightly coupled. This coupling determines the following
characteristics:
o The details of the topology and routing information advertised by
the optical network across the client interface;
o The level of control that IP routers can exercise in selecting
explicit paths for connections across the optical network;
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RFC 3717 IP over Optical Networks: A Framework March 2004
o Policies regarding the dynamic provisioning of optical paths
between routers. These include access control, accounting, and
security issues.
The following interconnection models are then possible:
5.1. Interconnection Models5.1.1. The Peer Model
Under the peer model, the IP control plane acts as a peer of the
optical transport network control plane. This implies that a single
instance of the control plane is deployed over the IP and optical
domains. When there is a single optical network involved and the IP
and optical domains belong to the same entity, then a common IGP such
as OSPF or IS-IS, with appropriate extensions, can be used to
distribute topology information [7] over the integrated IP-optical
network. In the case of OSPF, opaque LSAs can be used to advertise
topology state information. In the case of IS-IS, extended TLVs will
have to be defined to propagate topology state information. Many of
these extensions are occurring within the context of GMPLS.
When an optical internetwork with multiple optical networks is
involved (e.g., spanning different administrative domains), a single
instance of an intra-domain routing protocol is not attractive or
even realistic. In this case, inter-domain routing and signaling
protocols are needed. In either case, a tacit assumption is that a
common addressing scheme will be used for the optical and IP
networks. A common address space can be trivially realized by using
IP addresses in both IP and optical domains. Thus, the optical
network elements become IP addressable entities as noted in [1].
5.1.2. The Overlay Model
Under the overlay model, the IP layer routing, topology distribution,
and signaling protocols are independent of the routing, topology
distribution, and signaling protocols within the optical domain.
This model is conceptually similar to the classical IP over ATM or
MPOA models, but applied to an optical internetwork instead. In the
overlay model, a separate instance of the control plane (especially
the routing and signaling protocols) would have to be deployed in the
optical domain, independent of what exists in the IP domain. In
certain circumstances, it may also be feasible to statically
configure the optical channels that provide connectivity for the IP
domain in the overlay model. Static configuration can be effected
through network management functions. Static configuration, however,
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RFC 3717 IP over Optical Networks: A Framework March 2004
is unlikely to scale in very large networks, and may not support the
rapid connection provisioning requirements of future highly
competitive networking environments.
5.1.3. The Augmented Model
Under the augmented model, there are separate routing instances in
the IP and optical domains, but certain types of information from one
routing instance can be passed through to the other routing instance.
For example, external IP addresses could be carried within the
optical routing protocols to allow reachability information to be
passed to IP clients.
The routing approaches corresponding to these interconnection models
are described below.
5.2. Routing Approaches5.2.1. Integrated Routing
This routing approach supports the peer model within a single
administrative domain. Under this approach, the IP and optical
networks are assumed to run the same instance of an IP routing
protocol, e.g., OSPF with suitable "optical" extensions. These
extensions must capture optical link parameters, and any constraints
that are specific to optical networks. The topology and link state
information maintained by all nodes (OXCs and routers) may be
identical, but not necessarily. This approach permits a router to
compute an end-to-end path to another router across the optical
network. Suppose the path computation is triggered by the need to
route a label switched path (LSP) in a GMPLS environment. Such an
LSP can be established using GMPLS signaling, e.g., RSVP-TE or CR-LDP
with appropriate extensions. In this case, the signaling protocol
will establish a lightpath between two edge routers. This lightpath
is in essence a tunnel across the optical network, and may have
capacity much larger than the bandwidth required to support the first
LSP. Thus, it is essential that other routers in the network realize
the availability of excess capacity within the lightpath so that
subsequent LSPs between the routers can use it rather than
instantiating a new lightpath. The lightpath may therefore be
advertised as a virtual link in the topology as a means to address
this issue.
The notion of "forwarding adjacency" (FA) described in [3] is
essential in propagating existing lightpath information to other
routers. An FA is essentially a virtual link advertised into a link
state routing protocol. Thus, an FA could be described by the same
parameters that define resources in any regular link. While it is
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necessary to specify the mechanism for creating an FA, it is not
necessary to specify how an FA is used by the routing scheme. Once
an FA is advertised in a link state protocol, its usage for routing
LSPs is defined by the route computation and traffic engineering
algorithms implemented.
It should be noted that at the IP-optical interface, the physical
ports over which routers are connected to OXCs constrain the
connectivity and resource availability. Suppose a router R1 is
connected to OXC O1 over two ports, P1 and P2. Under integrated
routing, the connectivity between R1 and O1 over the two ports would
have been captured in the link state representation of the network.
Now, suppose an FA at full port bandwidth is created from R1 to
another router R2 over port P1. While this FA is advertised as a
virtual link between R1 and R2, it is also necessary to remove the
link R1-O1 (over P1) from the link state representation since that
port is no longer available for creating a lightpath. Thus, as FAs
are created, an overlaid set of virtual links is introduced into the
link state representation, replacing the links previously advertised
at the IP-Optical interface. Finally, the details of the optical
network captured in the link state representation is replaced by a
network of FAs. The above scheme is one way to tackle the problem.
Another approach is to associate appropriate dynamic attributes with
link state information, so that a link that cannot be used to
establish a particular type of connection will be appropriately
tagged. Generally, however, there is a great deal of similarity
between integrated routing and domain-specific routing (described
next). Both ultimately deal with the creation of a virtual
lightpath topology (which is overlaid over the optical network) to
meet certain traffic engineering objectives.
5.2.2. Domain-Specific Routing
The domain-specific routing approach supports the augmented
interconnection model. Under this approach, routing within the
optical and IP domains are separated, with a standard routing
protocol running between domains. This is similar to the IP inter-
domain routing model. A specific approach for this is considered
next. It is to be noted that other approaches are equally possible.
5.2.2.1. Domain-Specific Routing using BGP
The inter-domain IP routing protocol, BGP [8], may be adapted for
exchanging routing information between IP and optical domains. This
would allow routers to advertise IP address prefixes within their
network to the optical internetwork and to receive external IP
address prefixes from the optical internetwork. The optical
internetwork transports the reachability information from one IP
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network to others. For instance, edge routers and OXCs can run
exterior BGP (EBGP). Within the optical internetwork, interior BGP
(IBGP) is may be used between border optical switches, and EBGP may
be used between different networks (over ENNI, Figure 1).
Under this scheme, it may be necessary to identify the egress points
in the optical internetwork corresponding to externally reachable IP
addresses. To see this, suppose an edge router intends to establish
an LSP to a destination node across the optical internetwork. It may
request a direct lightpath to that destination, without explicitly
specifying the egress optical port for the lightpath because the
optical internetwork has knowledge of externally reachable IP
addresses. However, if the same edge router were to establish
another LSP to a different external destination, then for efficiency
reasons, it may first need to determine whether there is an existing
lightpath (with sufficient residual capacity) to the target
destination. For this purpose, it may be necessary for edge routers
to keep track of which egress ports in the optical internetwork lead
to which external destinations. Thus, a border OXC receiving
external IP prefixes from an edge router through EBGP must include
its own IP address as the egress point before propagating these
prefixes to other border OXCs or edge routers. An edge router
receiving this information need not propagate the egress address
further, but it must keep the association between external IP
addresses and egress OXC addresses. When optical VPNs are
implemented, the address prefixes advertised by the border OXCs may
be accompanied by some VPN specific identification.
There are however, some potential negative effects that could result
from domain-specific routing using BGP in an IPO environment:
o The amount of information that optical nodes will have to maintain
will not be bound by the size of the optical network anymore, but
will have to include external routes as well.
o The stability of the optical network control plane will no longer
be dictated solely by the dynamics emanating within the optical
network, but may be affected by the dynamics originating from
external routing domains from which external reachability
information is received.
5.2.3. Overlay Routing
The overlay routing approach supports the overlay interconnection
model. Under this approach, an overlay mechanism that allows edge
routers to register and query for external addresses is implemented.
This is conceptually similar to the address resolution mechanism used
for IP over ATM. Under this approach, the optical network could
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implement a registry that allows edge routers to register IP
addresses and VPN identifiers. An edge router may be allowed to
query for external addresses belonging to the same set of VPNs it
belongs to. A successful query would return the address of the
egress optical port through which the external destination can be
reached.
Because IP-optical interface connectivity is limited, the
determination of how many lightpaths must be established and to what
endpoints are traffic engineering decisions. Furthermore, after an
initial set of such lightpaths are established, these may be used as
adjacencies within VPNs for a VPN-wide routing scheme, for example,
OSPF. With this approach, an edge router could first determine other
edge routers of interest by querying the registry. After it obtains
the appropriate addresses, an initial overlay lightpath topology may
be formed. Routing adjacencies may then be established across the
lightpaths and further routing information may be exchanged to
establish VPN-wide routing.
5.3. Signaling-Related5.3.1. The Role of MPLS
It is possible to model wavelengths, and potentially TDM channels
within a wavelength as "labels". This concept was proposed in [1],
and "generalized" MPLS (GMPLS) mechanisms for realizing this are
described in [4]. MPLS signaling protocols with traffic engineering
extensions, such as RSVP-TE, can be appropriately extended and used
for signaling lightpath requests. These protocols can be adapted for
client/server signaling in the case of the domain services model, and
for end-to-end integrated signaling in the case of the unified
services model.
5.3.2. Signaling Models
With the domain-services model, the signaling control plane in the IP
and optical network are completely separate as shown in Figure 3
below. This separation also implies the separation of IP and optical
address spaces (even though the optical network would be using
internal IP addressing). While RSVP-TE and LDP can be adapted for
UNI signaling, the full functionality of these protocols will not be
used. For example, UNI signaling does not require the specification
of explicit routes. On the other hand, based on the service
attributes, new objects need to be signaled using these protocols as
described in [5, 6].
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MPLS Signaling UNI Signaling MPLS or other signaling
|
+-----------------------------+ | +-----------------------------+
| IP Network | | | Optical Internetwork |
| +---------+ +---------+ | | | +---------+ +---------+ |
| | | | | | | | | | | | |
| | Router +---+ Router +-----+------+ OXC +---+ OXC | |
| | | | | | | | | | | | |
| +-----+---+ +---+-----+ | | | +-----+---+ +---+-----+ |
+-----------------------------+ | +-----------------------------+
|
|
Completely Separated Addressing and Control Planes
Figure 3: Domain Services Signaling Model
With the unified services model, the addressing is common in the IP
network and optical internetwork and the respective signaling control
are related, as shown in Figure 4. It is understood that GMPLS
signaling is implemented in the IP and optical domains, using
suitably enhanced RSVP-TE or CR-LDP protocols. But the semantics of
services within the optical internetwork may be different from that
in the IP network. As an example, the protection services offered in
the optical internetwork may be different from the end-to-end
protection services offered by the IP network. Another example is
with regard to bandwidth. While the IP network may offer a continuum
of bandwidths, the optical internetwork will offer only discrete
bandwidths. Thus, the signaling attributes and services are defined
independently for IP and optical domains. The routers at the edge of
the optical internetwork must therefore identify service boundaries
and perform suitable translations in the signaling messages crossing
the IP-optical boundary. This may still occur even though the
signaling control plane in both networks are GMPLS-based and there is
tighter coupling of the control plane as compared to the domain
services model.
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Service Boundary Service Boundary
| |
IP Layer GMPLS Signaling | Optical Layer GMPLS | IP Layer GMPLS
| |
+--------+ +--------+ | +-------+ +-------+ | +--------+
| | | | | | | | | | | |
| IP LSR +--+ IP LSR +--+--+Optical+--+Optical+-+--+ IP LSR +---
| | | | | | LSR | | LSR | | | |
+-----+--+ +---+----+ | +-----+-+ +---+---+ | +--------+
Common Address Space, Service Translation
Figure 4: Unified Services Signaling Model
Thus, as illustrated in Figure 4, the signaling in the case of
unified services is actually multi-layered. The layering is based on
the technology and functionality. As an example, the specific
adaptations of GMPLS signaling for SONET layer (whose functionality
is transport) are described in [10].
5.4. End-to-End Protection Models
Suppose an LSP is established from an ingress IP router to an egress
router across an ingress IP network, a transit optical internetwork
and an egress IP network. If this LSP is to be afforded protection
in the IP layer, how is the service coordinated between the IP and
optical layers?
Under this scenario, there are two approaches to end-to-end
protection:
5.4.1. Segment-Wise Protection
The protection services in the IP layer could utilize optical layer
protection services for the LSP segment that traverses the optical
internetwork. Thus, the end-to-end LSP would be treated as a
concatenation of three LSP segments from the protection point of
view: a segment in the ingress IP network, a segment in the optical
internetwork and a segment in the egress IP network. The protection
services at the IP layer for an end-to-end LSP must be mapped onto
suitable protection services offered by the optical internetwork.
Suppose that 1+1 protection is offered to LSPs at the IP layer, i.e.,
each protected LSP has a pre-established hot stand-by in a 1+1 or 1:1
configuration. In case of a failure of the primary LSP, traffic can
be immediately switched to the stand-by. This type of protection can
be realized end-to-end as follows. With reference to Figure 5, let
an LSP originate at (ingress) router interface A and terminate at
(egress) router interface F. Under the first protection option, a
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primary path for the LSP must be established first. Let this path be
as shown in Figure 5, traversing router interface B in the ingress
network, optical ports C (ingress) and D (egress), and router
interface E in the egress network. Next, 1+1 protection is realized
separately in each network by establishing a protection path between
points A and B, C and D and E and F. Furthermore, the segments B-C
and D-E must themselves be 1+1 protected, using drop- side
protection. For the segment between C and D, the optical
internetwork must offer a 1+1 service similar to that offered in the
IP networks.
+----------------+ +------------------+ +---------------+
| | | | | |
A Ingress IP Net B----C Optical Internet D----E Egress IP Net F
| | | | | |
+----------------+ +------------------+ +---------------+
Figure 5: End-to-End Protection Example
5.4.2. Single-Layer Protection
Under this model, the protection services in the IP layer do not rely
on any protection services offered in the optical internetwork. Thus,
with reference to Figure 5, two SRLG-disjoint LSPs are established
between A and F. The corresponding segments in the optical
internetwork are treated as independent lightpaths in the optical
internetwork. These lightpaths may be unprotected in the optical
internetwork.
5.4.3. Differences
A distinction between these two choices is as follows. Under the
first choice, the optical internetwork is actively involved in end-
to-end protection, whereas under the second choice, any protection
service offered in the optical internetwork is not utilized directly
by client IP network. Also, under the first choice, the protection
in the optical internetwork may apply collectively to a number of IP
LSPs. That is, with reference to Figure 5, many LSPs may be
aggregated into a single lightpath between C and D. The optical
internetwork protection may then be applied to all of them at once
leading to some gain in scalability. Under the second choice, each
IP LSP must be separately protected. Finally, the first choice
allows different restoration signaling to be implemented in the IP
and optical internetwork. These restoration protocols are "patched
up" at the service boundaries to realize end-to-end protection. A
further advantage of this is that restoration is entirely contained
within the network where the failure occurs, thereby improving the
restoration latency, and perhaps network stability as a fault within
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an optical domain is contained and corrected within the domain. For
instance, if there is a failure in the optical internetwork, optical
network protocols restore the affected internal segments. Under the
second choice, restoration signaling is always end-to-end between IP
routers, essentially by-passing the optical internetwork. A result
of this is that restoration latency could be higher. In addition,
restoration protocols in the IP layer must run transparently over the
optical internetwork in the overlay mode. IP based recovery
techniques may however be more resource efficient, as it may be
possible to convey traffic through the redundant capacity under
fault-free scenarios. In particular, it may be possible to utilize
classification, scheduling, and concepts of forwarding equivalence
class to route lower class traffic over protect facilities and then
possibly preempt them to make way for high priority traffic when
faults occur.
6. IP-based Optical Control Plane Issues
Provisioning and restoring lightpaths end-to-end between IP networks
requires protocol and signaling support within optical sub-networks,
and across the INNI and ENNI. In this regard, a distinction is made
between control procedures within an optical sub-network (Figure 1),
between sub-networks, and between networks. The general guideline
followed in this framework is to separate these cases, and allow the
possibility that different control procedures are followed inside
different sub-networks, while a common set of procedures are followed
across sub-networks and networks.
The control plane procedures within a single vendor sub-network need
not be defined since these can be proprietary. Clearly, it is
possible to follow the same control procedures inside a sub-network
and across sub-networks. But this is simply a recommendation within
this framework document, rather than an imperative requirement. Thus,
in the following, signaling and routing across sub-networks is
considered first, followed by a discussion of similar issues across
networks.
6.1. Addressing
For interoperability across optical sub-networks using an IP-centric
control plane, one of the fundamental issues is that of addressing.
What entities should be identifiable from a signaling and routing
point of view? How should they be addressed? This section presents
some high level guidelines on this issue.
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Identifiable entities in optical networks include OXCs, optical
links, optical channels and sub-channels, Shared Risk Link Groups
(SRLGs), etc. An issue here is how granular the identification
should be as far as the establishment of optical trails are
concerned. The scheme for identification must accommodate the
specification of the termination points in the optical network with
adequate granularity when establishing optical trails. For instance,
an OXC could have many ports, each of which may in turn terminate
many optical channels, each of which contain many sub-channels etc.
It is perhaps not reasonable to assume that every sub-channel or
channel termination, or even OXC ports could be assigned a unique IP
address. Also, the routing of an optical trail within the network
does not depend on the precise termination point information, but
rather only on the terminating OXC. Thus, finer granularity
identification of termination points is of relevance only to the
terminating OXC and not to intermediate OXCs (of course, resource
allocation at each intermediate point would depend on the granularity
of resources requested). This suggests an identification scheme
whereby OXCs are identified by a unique IP address and a "selector"
identifies further fine-grain information of relevance at an OXC.
This, of course, does not preclude the identification of these
termination points directly with IP addresses(with a null selector).
The selector can be formatted to have adequate number of bits and a
structure that expresses port, channel, sub-channel, etc,
identification.
Within the optical network, the establishment of trail segments
between adjacent OXCs require the identification of specific port,
channel, sub-channel, etc. With a GMPLS control plane, a label
serves this function. The structure of the label must be such that
it can encode the required information [10].
Another entity that must be identified is the SRLG [11]. An SRLG is
an identifier assigned to a group of optical links that share a
physical resource. For instance, all optical channels routed over
the same fiber could belong to the same SRLG. Similarly, all fibers
routed over a conduit could belong to the same SRLG. The notable
characteristic of SRLGs is that a given link could belong to more
than one SRLG, and two links belonging to a given SRLG may
individually belong to two other SRLGs. This is illustrated in
Figure 6. Here, the links 1,2,3 and 4 may belong to SRLG 1, links
1,2 and 3 could belong to SRLG 2 and link 4 could belong to SRLG 3.
Similarly, links 5 and 6 could belong to SRLG 1, and links 7 and 8
could belong to SRLG 4. (In this example, the same SRLG, i.e., 1,
contains links from two different adjacencies).
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While the classification of physical resources into SRLGs is a manual
operation, the assignment of unique identifiers to these SRLGs
within an optical network is essential to ensure correct SRLG-
disjoint path computation for protection. SRLGs could be identified
with a flat identifier (e.g., 32 bit integer).
Finally, optical links between adjacent OXCs may be bundled for
advertisement into a link state protocol [12]. A bundled interface
may be numbered or unnumbered. In either case, the component links
within the bundle must be identifiable. In concert with SRLG
identification, this information is necessary for correct path
computation.
6.2. Neighbor Discovery
Routing within the optical network relies on knowledge of network
topology and resource availability. This information may be gathered
and used by a centralized system, or by a distributed link state
routing protocol. In either case, the first step towards network-
wide link state determination is the discovery of the status of local
links to all neighbors by each OXC. Specifically, each OXC must
determine the up/down status of each optical link, the bandwidth and
other parameters of the link, and the identity of the remote end of
the link (e.g., remote port number). The last piece of information
is used to specify an appropriate label when signaling for lightpath
provisioning. The determination of these parameters could be based
on a combination of manual configuration and an automated protocol
running between adjacent OXCs. The characteristics of such a
protocol would depend on the type of OXCs that are adjacent (e.g.,
transparent or opaque).
Neighbor discovery would typically require in-band communication on
the bearer channels to determine local connectivity and link status.
In the case of opaque OXCs with SONET termination, one instance of a
neighbor discovery protocol (e.g., LMP [2]) would run on each OXC
port, communicating with the corresponding protocol instance at the
neighboring OXC. The protocol would utilize the SONET overhead bytes
to transmit the (configured) local attributes periodically to the
neighbor. Thus, two neighboring switches can automatically determine
the identities of each other and the local connectivity, and also
keep track of the up/down status of local links. Neighbor discovery
with transparent OXCs is described in [2].
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+--------------+ +------------+ +------------+
| +-1:OC48---+ +-5:OC192-+ |
| +-2:OC48---+ +-6:OC192-+ |
| OXC1 +-3:OC48---+ OXC2 +-7:OC48--+ OXC3 |
| +-4:OC192--+ +-8:OC48--+ |
| | | | +------+ |
+--------------+ +----+-+-----+ | +----+------+-----+
| | | | |
| | | | |
+--------------+ | | | | |
| | +----+-+-----+ | | +------+-----+
| +----------+ +--+ | | |
| OXC4 +----------+ +----+ | |
| +----------+ OXC5 +--------+ OXC6 |
| | | +--------+ |
+--------------+ | | | |
+------+-----+ +------+-----+
Figure 6: Mesh Optical Network with SRLGs
6.3. Topology Discovery
Topology discovery is the procedure by which the topology and
resource state of all the links in a network are determined. This
procedure may be done as part of a link state routing protocol (e.g.,
OSPF, ISIS), or it can be done via the management plane (in the case
of centralized path computation). The implementation of a link state
protocol within a network (i.e., across sub-network boundaries) means
that the same protocol runs in OXCs in every sub-network. If this
assumption does not hold then interworking of routing between sub-
networks is required. This is similar to inter-network routing
discussed in Section 6.7. The focus in the following is therefore on
standardized link state routing.
In general, most of the link state routing functionality is
maintained when applied to optical networks. However, the
representation of optical links, as well as some link parameters, are
changed in this setting. Specifically,
o The link state information may consist of link bundles [12]. Each
link bundle is represented as an abstract link in the network
topology. Different bundling representations are possible. For
instance, the parameters of the abstract link may include the
number, bandwidth and the type of optical links contained in the
underlying link bundle [12]. Also, the SRLGs corresponding to
each optical link in the bundle may be included as a parameter.
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o The link state information should capture restoration-related
parameters for optical links. Specifically, with shared
protection (Section 6.5), the link state updates must have
information that allows the computation of shared protection
paths.
o A single routing adjacency could be maintained between neighbors
which may have multiple optical links (or even multiple link
bundles) between them. This reduces the protocol messaging
overhead.
o Since link availability information changes dynamically, a
flexible policy for triggering link state updates based on
availability thresholds may be implemented. For instance, changes
in availability of links of a given bandwidth (e.g., OC-48) may
trigger updates only after the availability figure changes by a
certain percentage.
These concepts are relatively well-understood. On the other hand,
the resource representation models and the topology discovery process
for hierarchical routing (e.g., OSPF with multiple areas) are areas
that need further work.
6.4. Protection and Restoration Models
Automatic restoration of lightpaths is a service offered by optical
networks. There could be local and end-to-end mechanisms for
restoration of lightpaths within a network (across the INNI). Local
mechanisms are used to select an alternate link (or network segment)
between two OXCs across the INNI when a failure affects the primary
link (or primary network segment) over which the (protected)
lightpath is routed. Local restoration does not affect the end-to-
end route of the lightpath. When local restoration is not possible
(e.g., no alternate link is available between the adjacent OXCs in
question), end-to-end restoration may be performed. Under this
scenario this, the affected lightpath may be rerouted over an
alternate diverse path to circumvent failed resources. For end-to-
end restoration, alternate paths may be pre-computed to expedite the
recovery time. End to end restoration may also be mixed with local
recovery in various ways depending on acceptable tradeoffs between
utilization of network resources and recovery times.
End-to-end protection may be based on two types of protection
schemes; "1 + 1" protection or shared protection. Under 1 + 1
protection, a back-up path is established for the protected primary
path along a physically diverse route. Both paths are active and the
failure along the primary path results in an immediate switch-over to
the back-up path. Under shared protection, back-up paths
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corresponding to physically diverse primary paths may share the same
network resources. When a failure affects a primary path, it is
assumed that the same failure will not affect the other primary paths
whose back-ups share resources.
It is possible that different restoration schemes may be implemented
within optical sub-networks. It is therefore necessary to consider a
two-level restoration mechanism. Path failures within an optical
sub-network could be handled using procedures specific to the sub-
network. If this fails, end-to-end restoration across sub-networks
could be invoked. The border OXC that is the ingress to a sub-
network can act as the source for restoration procedures within a
sub-network. The signaling for invoking end-to-end restoration
across the INNI is described in Section 6.6.3. The computation of
the back-up path for end-to-end restoration may be based on various
criteria. It is assumed that the back-up path is computed by the
source OXC, and signaled using standard methods.
6.5. Route Computation
The computation of a primary route for a lightpath within an optical
network is essentially a constraint-based routing problem. The
constraint is typically the bandwidth required for the lightpath,
perhaps along with administrative and policy constraints. The
objective of path computation could be to minimize the total capacity
required for routing lightpaths [13].
Route computation with constraints may be accomplished using a number
of algorithms [14]. When 1+1 protection is used, a back-up path that
does not traverse on any link which is part of the same SRLG as links
in the primary path must be computed. Thus, it is essential that the
SRLGs in the primary path be known during alternate path computation,
along with the availability of resources in links that belong to
other SRLGs. This requirement has certain implications on optical
link bundling. Specifically, a bundled LSA must include adequate
information such that a remote OXC can determine the resource
availability under each SRLG that the bundled link refers to, and the
relationship between links belonging to different SRLGs in the
bundle. For example, considering Figure 3, if links 1,2,3 and 4 are
bundled together in an LSA, the bundled LSA must indicate that there
are three SRLGs which are part of the bundle (i.e., 1, 2 and 3), and
that links in SRLGs 2 and 3 are also part of SRLG 1.
To encode the SRLG relationships in a link bundle LSA, only links
which belong to exactly the same set of SRLGs must be bundled
together. With reference to Figure 3, for example, two bundles can
be advertised for links between OXC1 and OXC2, with the following
information:
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Bundle No. SRLGs Link Type Number Other Info
-------------------------------------------------------
1 1,2 OC-48 3 ---
2 1,3 OC-192 1 ---
Assuming that the above information is available for each bundle at
every node, there are several approaches possible for path
computation. For instance,
1. The primary path can be computed first, and the (exclusive or
shared) back-up is computed next based on the SRLGs chosen for the
primary path. In this regard,
o The primary path computation procedure can output a series of
bundles the path is routed over. Since a bundle is uniquely
identified with a set of SRLGs, the alternate path can be
computed right away based on this knowledge. In this case, if
the primary path set up does not succeed for lack of resources
in a chosen bundle, the primary and backup paths must be
recomputed.
o It might be desirable to compute primary paths without choosing
a specific bundle apriori. That is, resource availability over
all bundles between a node pair is taken into account rather
than specific bundle information. In this case, the primary
path computation procedure would output a series of nodes the
path traverses. Each OXC in the path would have the freedom to
choose the particular bundle to route that segment of the
primary path. This procedure would increase the chances of
successfully setting up the primary path when link state
information is not up to date everywhere. But the specific
bundle chosen, and hence the SRLGs in the primary path, must be
captured during primary path set-up, for example, using the
RSVP-TE Route Record Object [15]. This SRLG information is
then used for computing the back-up path. The back-up path may
also be established specifying only which SRLGs to avoid in a
given segment, rather than which bundles to use. This would
maximize the chances of establishing the back-up path.
2. The primary path and the back-up path are computed together in one
step, for example, using Suurbaale's algorithm [16]. In this
case, the paths must be computed using specific bundle
information.
To summarize, it is essential to capture sufficient information in
link bundle LSAs to accommodate different path computation procedures
and to maximize the chances of successful path establishment.
Depending on the path computation procedure used, the type of support
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needed during path establishment (e.g., the recording of link group
or SRLG information during path establishment) may differ.
When shared protection is used, the route computation algorithm must
take into account the possibility of sharing links among multiple
back-up paths. Under shared protection, the back-up paths
corresponding to SRLG-disjoint primary paths can be assigned the same
links. The assumption here is that since the primary paths are not
routed over links that have the same SRLG, a given failure will
affect only one of them. Furthermore, it is assumed that multiple
failure events affecting links belonging to more than one SRLG will
not occur concurrently. Unlike the case of 1+1 protection, the
back-up paths are not established apriori. Rather, a failure event
triggers the establishment of a single back-up path corresponding to
the affected primary path.
The distributed implementation of route computation for shared back-
up paths require knowledge about the routing of all primary and
back-up paths at every node. This raises scalability concerns. For
this reason, it may be practical to consider the centralization of
the route computation algorithm in a route server that has complete
knowledge of the link state and path routes. Heuristics for fully
distributed route computation without complete knowledge of path
routes are to be determined. Path computation for restoration is
further described in [11].
6.6. Signaling Issues
Signaling within an optical network for lightpath provisioning is a
relatively simple operation if a standard procedure is implemented
within all sub-networks. Otherwise, proprietary signaling may be
implemented within sub-networks, but converted back to standard
signaling across the INNI. This is similar to signaling across the
ENNI, as described in Section 6.7. In the former case, signaling
messages may carry strict explicit route information, while in the
latter case the route information should be loose, at the level of
abstraction of sub-networks. Once a route is determined for a
lightpath, each OXC along the path must appropriately configure their
cross-connects in a coordinated fashion. This coordination is
conceptually analogous to selecting incoming and outgoing labels in a
label-switched environment. Thus, protocols like RSVP-TE [9] may be
adapted and used across the INNI for this purpose. The adaptation of
IP-based signaling protocols must take into account a number of
peculiar attributes of optical networks.
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RFC 3717 IP over Optical Networks: A Framework March 20046.6.1. Bi-Directional Lightpath Establishment
Lightpaths are typically bi-directional. That is, the output port
selected at an OXC for the forward direction is also the input port
for the reverse direction of the path. Since signaling for optical
paths may be autonomously initiated by different nodes, it is
possible that two path set-up attempts are in progress at the same
time. Specifically, while setting up an optical path, an OXC A may
select output port i which is connected to input port j of the "next"
OXC B. Concurrently, OXC B may select output port j for setting up a
different optical path, where the "next" OXC is A. This results in a
"collision". Similarly, when WDM functionality is built into OXCs, a
collision occurs when adjacent OXCs choose directly connected output
ports and the same wavelength for two different optical paths. There
are two ways to deal with such collisions. First, collisions may be
detected and the involved paths may be torn down and re-established.
Or, collisions may be avoided altogether.
6.6.2. Failure Recovery
The impact of transient partial failures must be minimized in an
optical network. Specifically, optical paths that are not directly
affected by a failure must not be torn down due to the failure. For
example, the control processor in an OXC may fail, affecting
signaling and other internodal control communication. Similarly,
the control channel between OXCs may be affected temporarily by a
failure. These failure may not affect already established optical
paths passing through the OXC fabric. The detection of such failures
by adjacent nodes, for example, through a keepalive mechanism between
signaling peers, must not result in these optical paths being torn
down.
It is likely that when the above failures occur, a backup processor
or a backup control channel will be activated. The signaling
protocol must be designed such that it is resilient to transient
failures. During failure recovery, it is desirable to recover local
state at the concerned OXC with least disruption to existing optical
paths.
6.6.3. Restoration
Signaling for restoration has two distinct phases. There is a
reservation phase in which capacity for the protection path is
established. Then, there is an activation phase in which the back-up
path is actually put in service. The former phase typically is not
subject to strict time constraints, while the latter is.
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RFC 3717 IP over Optical Networks: A Framework March 2004
Signaling to establish a "1+1" back-up path is relatively straight-
forward. This signaling is very similar to signaling used for
establishing the primary path. Signaling to establish a shared
back-up path is a little bit different. Here, each OXC must
understand which back-up paths can share resources among themselves.
The signaling message must itself indicate shared reservation. The
sharing rule is as described in Section 6.4: back-up paths
corresponding to physically diverse primary paths may share the same
network resources. It may therefore be necessary for the signaling
message to carry adequate information that allows an OXC to verify
that appropriateness of having a set of back-up paths sharing
certain.
Under both 1+1 and shared protection, the activation phase has two
parts: propagation of failure information to the source OXC from the
point of failure, and activation of the back-up path. The signaling
for these two phases must be very fast in order to realize response
times in the order of tens of milliseconds. When optical links are
SONET-based, in-band signals may be used, resulting in expedited
response. With out-of-band control, it may be necessary to consider
fast signaling over the control channel using very short IP packets
and prioritized processing. While it is possible to use RSVP or CR-
LDP for activating protection paths, these protocols do not provide
any means to give priority to restoration signaling as opposed to
signaling for provisioning. For instance, it is possible for a
restoration-related RSVP message to be queued behind a number of
provisioning messages thereby delaying restoration. It may therefore
be necessary to develop a notion of prioritization for restoration
signaling and incorporate appropriate mechanisms into existing
signaling protocols to achieve this. Alternatively, a new signaling
mechanism may be developed exclusively for activating protection
paths during restoration.
6.7. Optical Internetworking
Within an optical internetwork, it must be possible to dynamically
provision and restore lightpaths across optical networks. Therefore:
o A standard scheme for uniquely identifying lightpath end-points in
different networks is required.
o A protocol is required for determining reachability of end-points
across networks.
o A standard signaling protocol is required for provisioning
lightpaths across networks.
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RFC 3717 IP over Optical Networks: A Framework March 2004
o A standard procedure is required for the restoration of lightpaths
across networks.
o Support for policies that affect the flow of control information
across networks will be required.
The IP-centric control architecture for optical networks can be
extended to satisfy the functional requirements of optical
internetworking. Routing and signaling interaction between optical
networks can be standardized across the ENNI (Figure 1). The
functionality provided across ENNI is as follows.
6.7.1. Neighbor Discovery
Neighbor discovery procedure, as described in Section 6.2, can be
used for this. Indeed, a single protocol should be standardized for
neighbor discovery within and across networks.
6.7.2. Addressing and Routing Model
The addressing mechanisms described in Section 6.1 can be used to
identify OXCs, ports, channels and sub-channels in each network. It
is essential that the OXC IP addresses are unique within the
internetwork.
Provisioning an end-to-end lightpath across multiple networks
involves the establishment of path segments in each network
sequentially. Thus, a path segment is established from the source
OXC to a border OXC in the source network. From this border OXC,
signaling across NNI is used to establish a path segment to a border
OXC in the next network. Provisioning then continues in the next
network and so on until the destination OXC is reached. The usage of
protocols like BGP for this purpose need to be explored.
6.7.3. Restoration
Local restoration across the ENNI is similar to that across INNI
described in Section 6.6.3. End-to-end restoration across networks
is likely to be either of the 1+1 type, or segmented within each
network, as described in Section 6.4.
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RFC 3717 IP over Optical Networks: A Framework March 20047. Other Issues7.1. WDM and TDM in the Same Network
A practical assumption would be that if SONET (or some other TDM
mechanism that is capable partitioning the bandwidth of a wavelength)
is used, then TDM is leveraged as an additional method to
differentiate between "flows". In such cases, wavelengths and time
intervals (sub-channels) within a wavelength become analogous to
labels (as noted in [1]) which can be used to make switching
decisions. This would be somewhat akin to using VPI (e.g.,
wavelength) and VCI (e.g., TDM sub-channel) in ATM networks. More
generally, this will be akin to label stacking and to LSP nesting
within the context of Multi-Protocol Lambda Switching [1]. GMPLS
signaling [4] supports this type of multiplexing.
7.2. Wavelength Conversion
Some form of wavelength conversion may exist at some switching
elements. This however may not be the case in some pure optical
switching elements. A switching element is essentially anything more
sophisticated than a simple repeater, that is capable of switching
and converting a wavelength Lambda(k) from an input port to a
wavelength Lambda(l) on an output port. In this display, it is not
necessarily the case that Lambda(k) = Lambda(l), nor is it
necessarily the case that the data carried on Lambda(k) is switched
through the device without being examined or modified.
It is not necessary to have a wavelength converter at every switching
element. A number of studies have attempted to address the issue of
the value of wavelength conversion in an optical network. Such
studies typically use the blocking probability (the probability that
a lightpath cannot be established because the requisite wavelengths
are not available) as a metric to adjudicate the effectiveness of
wavelength conversion. The IP over optical architecture must take
into account hybrid networks with some OXCs capable of wavelength
conversion and others incapable of this. The GMPLS "label set"
mechanism [4] supports the selection of the same label (i.e.,
wavelength) across an NNI.
7.3. Service Provider Peering Points
There are proposed inter-network interconnect models which allow
certain types of peering relationships to occur at the optical layer.
This is consistent with the need to support optical layer services
independent of higher layers payloads. In the context of IP over
optical networks, peering relationships between different trust
domains will eventually have to occur at the IP layer, on IP routing
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RFC 3717 IP over Optical Networks: A Framework March 2004
elements, even though non-IP paths may exist between the peering
routers.
7.4. Rate of Lightpath Set-Up
Dynamic establishment of optical channel trails and lightpaths is
quite desirable in IP over optical networks, especially when such
instantiations are driven by a stable traffic engineering control
system, or in response to authenticated and authorized requests from
clients.
However, there are many proposals suggesting the use of dynamic,
data-driven shortcut-lightpath setups in IP over optical networks.
The arguments put forth in such proposals are quite reminiscent of
similar discussions regarding ATM deployment in the core of IP
networks. Deployment of highly dynamic data driven shortcuts within
core networks has not been widely adopted by carriers and ISPs for a
number of reasons: possible CPU overhead in core network elements,
complexity of proposed solutions, stability concerns, and lack of
true economic drivers for this type of service. This document
assumes that this paradigm will not change and that highly dynamic,
data-driven shortcut lightpath setups are for future investigation.
Instead, the optical channel trails and lightpaths that are expected
to be widely used at the initial phases in the evolution of IP over
optical networks will include the following:
o Dynamic connections for control plane traffic and default path
routed data traffic,
o Establishment and re-arrangement of arbitrary virtual topologies
over rings and other physical layer topologies.
o Use of stable traffic engineering control systems to engineer
lightpath connections to enhance network performance, either for
explicit demand based QoS reasons or for load balancing).
Other issues surrounding dynamic connection setup within the core
center around resource usage at the edge of the optical domain. One
potential issue pertains to the number of flows that can be processed
by an ingress or egress network element either because of aggregate
bandwidth limitations or because of a limitation on the number of
flows (e.g., lightpaths) that can be processed concurrently.
Another possible short term reason for dynamic shortcut lightpath
setup would be to quickly pre-provision paths based on some criteria
(e.g., a corporate executive wants a high bandwidth reliable
connection, etc.). In this scenario, a set of paths can be pre-
provisioned, but not actually instantiated until the customer
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RFC 3717 IP over Optical Networks: A Framework March 2004
initiates an authenticated and authorized setup requests, which is
consistent with existing agreements between the provider and the
customer. In a sense, the provider may have already agreed to supply
this service, but will only instantiate it by setting up a lightpath
when the customer submits an explicit request.
7.5. Distributed vs. Centralized Provisioning
This document has mainly dealt with a distributed model for lightpath
provisioning, in which all nodes maintain a synchronized topology
database, and advertise topology state information to maintain and
refresh the database. A constraint-based routing entity in each node
then uses the information in the topology database and other relevant
details to compute appropriate paths through the optical domain.
Once a path is computed, a signaling protocol (e.g., [9]) is used to
instantiate the lightpath.
Another provisioning model is to have a centralized server which has
complete knowledge of the physical topology, the available
wavelengths, and where applicable, relevant time domain information.
A corresponding client will reside on each network element that can
source or sink a lightpath. The source client would query the server
in order to set up a lightpath from the source to the destination.
The server would then check to see if such a lightpath can be
established based on prevailing conditions. Furthermore, depending
on the specifics of the model, the server may either setup the
lightpath on behalf of the client or provide the necessary
information to the client or to some other entity to allow the
lightpath to be instantiated.
Centralization aids in implementing complex capacity optimization
schemes, and may be the near-term provisioning solution in optical
networks with interconnected multi-vendor optical sub-networks. In
the long term, however, the distributed solution with centralization
of some control procedures (e.g., traffic engineering) is likely to
be the approach followed.
7.6. Optical Networks with Additional Configurable Components
Thus far, this memo has focused mainly on IP over optical networks
where the cross-connect is the basic dynamically re-configurable
device in the optical network. Recently, as a consequence of
technology evolution, various types of re-configurable optical
components are now available, including tunable lasers, tunable
filters, etc. Under certain circumstances, it may be necessary to
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RFC 3717 IP over Optical Networks: A Framework March 2004
parameterize the characteristics of these components and advertise
them within the control plane. This aspect is left for further
study.
7.7. Optical Networks with Limited Wavelength Conversion Capability
At the time of the writing of this document, the majority of optical
networks being deployed are "opaque". In this context the term
opaque means that each link is optically isolated by transponders
doing optical-electrical-optical conversions. Such conversions have
the added benefit of permitting 3R regeneration. The 3Rs refer to
re-power, signal retiming and reshaping. Unfortunately, this
regeneration requires that the underlying optical equipment be aware
of both the bit rate and frame format of the carried signal. These
transponders are quite expensive and their lack of transparency
constrains the rapid introduction of new services [17]. Thus there
are strong motivators to introduce "domains of transparency" wherein
all-optical networking equipment would transport data unfettered by
these drawbacks.
Thus, the issue of IP over optical networking in all optical sub-
networks, and sub-networks with limited wavelength conversion
capability merits special attention. In such networks, transmission
impairments resulting from the peculiar characteristics of optical
communications complicate the process of path selection. These
transmission impairments include loss, noise (due primarily to
amplifier spontaneous emission -- ASE), dispersion (chromatic
dispersion and polarization mode dispersion), cross-talk, and non-
linear effects. In such networks, the feasibility of a path between
two nodes is no longer simply a function of topology and resource
availability but will also depend on the accumulation of impairments
along the path. If the impairment accumulation is excessive, the
optical signal to noise ratio (OSNR) and hence the electrical bit
error rate (BER) at the destination node may exceed prescribed
thresholds, making the resultant optical channel unusable for data
communication. The challenge in the development of IP-based control
plane for optical networks is to abstract these peculiar
characteristics of the optical layer [17] in a generic fashion, so
that they can be used for path computation.
8. Evolution Path for IP over Optical Architecture
The architectural models described in Section 5 imply a certain
degree of implementation complexity. Specifically, the overlay model
was described as the least complex for near term deployment and the
peer model the most complex. Nevertheless, each model has certain
advantages and this raises the question as to the evolution path for
IP over optical network architectures.
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RFC 3717 IP over Optical Networks: A Framework March 2004
The evolution approach recommended in this framework is the
definition of capability sets that start with simpler functionality
in the beginning and include more complex functionality later. In
this regard, it is realistic to expect that initial IP over optical
deployments will be based on the domain services model (with overlay
interconnection), with no routing exchange between the IP and optical
domains. Under this model, direct signaling between IP routers and
optical networks is likely to be triggered by offline traffic
engineering decisions. The next step in the evolution of IP-optical
interaction is the introduction of reachability information exchange
between the two domains. This would potentially allow lightpaths to
be established as part of end-to-end LSP set-up. The final phase is
the support for the full peer model with more sophisticated routing
interaction between IP and optical domains.
Using a common signaling framework (based on GMPLS) from the
beginning facilitates this type of evolution. In this evolution, the
signaling capability and semantics at the IP-optical boundary would
become more sophisticated, but the basic structure of signaling would
remain. This would allow incremental developments as the
interconnection model becomes more sophisticated, rather than
complete re-development of signaling capabilities.
From a routing point of view, the use of Network Management Systems
(NMS) for static connection management is prevalent in legacy optical
networks. Going forward, it can be expected that connection routing
using the control plane will be gradually introduced and integrated
into operational infrastructures. The introduction of routing
capabilities can be expected to occur in a phased approach.
It is likely that in the first phase, service providers will either
upgrade existing local element management (EMS) software with
additional control plane capabilities (and perhaps the hardware as
well), or upgrade the NMS software in order to introduce some degree
of automation within each optical subnetwork. For this reason, it
may be desirable to partition the network into subnetworks and
introduce IGP interoperability within each subnetwork (i.e., at the
I-NNI level), and employ either static or signaled interoperability
between subnetworks. Consequently, it can be envisioned that the
first phase in the evolution towards network level control plane
interoperability in IP over Optical networks will be organized around
a system of optical subnetworks which are interconnected statically
(or dynamically in a signaled configuration). During this phase, an
overlay interconnection model will be used between the optical
network itself and external IP and MPLS routers (as described in
Section 5.2.3).
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RFC 3717 IP over Optical Networks: A Framework March 2004
Progressing with this phased approach to IPO routing
interoperabibility evolution, the next level of integration will be
achieved when a single carrier provides dynamic optical routing
interoperability between subnetworks and between domains. In order
to become completely independent of the network switching capability
within subnetworks and across domains, routing information exchange
may need to be enabled at the UNI level. This would constitute a
significant evolution: even if the routing instances are kept
separate and independent, it would still be possible to dynamically
exchange reachability and other types of routing information. Another
more sophisticated step during this phase is to introduce dynamic
routing at the E-NNI level. This means that any neighboring networks
(independent of internal switching capability) would be capable of
exchanging routing information with peers across the E-NNI.
Another alternative would be for private networks to bypass these
intermediate steps and directly consider an integrated routing model
from the onset. This direct evolution strategy is realistic, but is
more likely to occur in operational contexts where both the IP (or
MPLS) and optical networks are built simultaneously, using equipment
from a single source or from multiple sources that are closely
affiliated. In any case, due to the current lack of operational
experience in managing this degree of control plane interaction in a
heterogeneous network (these issues may exist even if the hardware
and software originate from the same vendor), an augmented model is
likely to be the most viable initial option. Alternatively, a very
modular or hierarchical peer model may be contemplated. There may be
other challenges (not just of a technical, but also administrative
and even political issues) that may need to be resolved in order to
achieve full a peer model at the routing level in a multi-technology
and multi-vendor environment. Ultimately, the main technical
improvement would likely arise from efficiencies derived from the
integration of traffic-engineering capabilities in the dynamic
inter-domain routing environments.
9. Security Considerations
The architectural framework described in this document requires a
number of different protocol mechanisms for its realization.
Specifically, the role of neighbor discovery, routing, and signaling
protocols were highlighted in previous sections. The general
security issues that arise with these protocols include:
o The authentication of entities exchanging information (e.g.,
signaling, routing, or link management) across a control
interface;
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RFC 3717 IP over Optical Networks: A Framework March 2004
o Ensuring the integrity of the information exchanged across the
interface;
o Protection of the control mechanisms from intrusions and other
modes of outside interference.
Because optical connections may carry high volumes of traffic and are
generally quite expensive, mechanisms are required to safeguard
optical networks against intrusions and unauthorized utilization of
network resources.
In addition to the security aspects relating to the control plane,
the data plane must also be protected from external interference.
An important consideration in optical networks is the separation of
control channels from data channels. This decoupling implies that
the state of the bearer channels carrying user traffic cannot be
inferred from the state of the control channels. Similarly, the
state of the control channels cannot be inferred from the state of
the data channels. The potential security implications of this
decoupling should be taken into account in the design of pertinent
control protocols and in the operation of IPO networks.
Another issue in IPO networks concerns the fact that the underlying
optical network elements may be invisible to IP client nodes,
especially in the overlay model. This means that traditional IP
tools such as traceroute cannot be used by client IP nodes to detect
attacks within the optical domain.
For the aforementioned reasons, the output of the routing protocol
security (RPSEC) efforts within the IETF should be considered in the
design of control protocols for optical networks.
In Section 2, the concept of a trust domain was defined as a network
under a single technical administration in which adequate security
measures are established to prevent unauthorized intrusion from
outside the domain. It should be strongly noted that within a trust
domain, any subverted node can send control messages which can
compromise the entire network.
9.1. General security aspects
Communication protocols usually require two main security mechanisms:
authentication and confidentiality. Authentication mechanisms ensure
data origin verification and message integrity so that intrusions and
unauthorized operations can be detected and mitigated. For example,
with reference to Figure 1, message authentication can prevent a
malicious IP client from mounting a denial of service attack against
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RFC 3717 IP over Optical Networks: A Framework March 2004
the optical network by invoking an excessive number of connection
creation requests across the UNI interface. Another important
security consideration is the need to reject replayed control
packets. This capability can assist in countering some forms of
denial of service attacks. Replay protection provides a form of
partial sequence integrity, and can be implemented in conjunction
with an authentication mechanism.
Confidentiality of signaling messages is also desirable, especially
in scenarios where message attributes between communicating entities
include sensitive or private information. Examples of such
attributes include account numbers, contract identification
information, and similar types of private data.
The case of equipment that are not co-located presents increased
security threats. In such scenarios, the communicating entities
engaged in protocol message transactions may be connected over an
external network. Generally, the external network may be outside the
span of control of the optical network (or client IP network)
administrators. As a result, the protocol messages may be subject to
increased security threats, such as address spoofing, eavesdropping,
and intrusion. To mitigate such threats, appropriate security
mechanisms must be employed to protect the control channels and
associated signaling and routing messages.
Requests for optical connections from client networks must also be
filtered using appropriate policies to protect against security
infringements and excess resource consumption. Additionally, there
may be a need for confidentiality of SRLGs in some circumstances.
Optical networks may also be subject to subtle forms of denial of
service attacks. An example of this would be requests for optical
connections with explicit routes that induce a high degree of
blocking for subsequent requests. This aspect might require some
global coordination of resource allocation.
Another related form of subtle denial of service attack could occur
when improbable optical paths are requested (i.e., paths within the
network for which resources are insufficiently provisioned). Such
requests for improbable paths may consume ports on optical switching
elements within the network resulting in denial of service for
subsequent connection requests.
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RFC 3717 IP over Optical Networks: A Framework March 20049.2. Security Considerations for Protocol Mechanisms
The security requirements for IP-centric control protocols employed
in the control plane of optical networks would depend on the specific
characteristics of the protocols and the security risks that exist in
a particular operational context. Such details relating to
particular operational contexts are beyond the scope of this document
and hence are not considered further. Nevertheless, it must be
stated that such control protocols must take into account the issues
associated with the separation of control channels from data channels
in switched optical networks, and the magnitude and extent of service
interruptions within the IP domain that could result from outages
emanating from the optical domain.
10. Summary and Conclusions
The objective of this document was to define a framework for IP over
optical networks, considering the service models, and routing and
signaling issues. There are a diversity of choices for IP-optical
control interconnection, service models, and protocol mechanisms. The
approach advocated in this document was to support different service
models which allow for future enhancements, and define complementary
signaling and routing mechanisms to enable these capabilities. An
evolutionary scenario, based on a common signaling framework (e.g.,
based on GMPLS) was suggested, with the capability to increase the
complexity of interworking functionality as the requirements become
more sophisticated. A key aspect of this evolutionary principle is
that the IP-optical control and service interaction is first based on
the domain services model with overlay interconnection that will
eventually evolve to support full peer interaction.
11. Informative References
[1] Awduche, D. and Y. Rekhter, "Multi-Protocol Lambda Switching:
Combining MPLS Traffic Engineering Control With Optical
Crossconnects", IEEE Communications Magazine, March 2001.
[2] Lang, J., et al., "Link Management Protocol", Work in progress.
[3] Kompella, K. and Y. Rekhter, "LSP Hierarchy with MPLS TE",
Internet Draft, Work in progress.
[4] Berger, L., Ed., "Generalized Multi-Protocol Label Switching
(GMPLS) Signaling Functional Description", RFC 3471, January
2003.
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RFC 3717 IP over Optical Networks: A Framework March 200415. Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78 and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
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Acknowledgement
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